Pneumolysin (Ply) has been identified in all pathogenic pneumococcal isolates. This 53,000 MW protein is a thiol-activated cytolysin. It belongs to a class of bacterial cytolysins including: perfringolysin, listeriolysin and alveolysin which are characterized by their ability to lyse Sheep red blood cells (SRBC). It has been proposed that an inactivated pneumolysin could provide a protein carrier for a pneumococcal polysaccharide conjugate vaccine. Monoclonal IgG and IgM antibodies to type 9V polysaccharide (PS) were produced in mice injected with type 9VPS conjugated to inactivated pneumolysin. Their binding activities to group 9 cross-reactive PSs were studied. The IgG and IgM monoclonal antibodies (MAb) were produced in mouse ascites fluid. A dilution of 1:2000 of the ascites fluid showed 9VPS antibodies of 0.45- 0.65 OD 405nm ELISA unit when incubated at 37 C for 1 hr. The inhibition of type 9V antigen-antibody reaction by cross-reactive group 9 PSs was examined. Pre-treatment with 9VPS absorbed most of 9V Ab and caused 72.9-75.0% inhibition of the 9VPS-9V MAb reaction. In the immunologic reaction using 9VPS MAb, 9APS induced 69.5% inhibition, 9LPS induced 19.2% and 9NPS caused almost no inhibition. In the reaction using rabbit 9V antiserum and 9VPS, the inhibition of the antigen-antibody reaction by 9LPS and 9NPS ranged from 57.8-62.7%. These cross-reactive PSs showed much higher inhibition in the reaction using rabbit antiserum. The type 9V IgG MAb stronly induced activation of PMN leukocytes for opsonophagocytic activity to kill 9V pneumococci.